1. Field of the Invention
The present invention relates to methods for operating a gas turbine plant.
In particular a gas turbine plant useful for implementing a method in accordance with principles of the present invention has a first and a second combustion chambers, such that hot gases are generated in the first combustion chamber and are expanded in a high pressure turbine; then these hot gases generated in the first combustion chamber are introduced into a second combustion chamber wherein further fuel is injected and combusted to generate second hot gases to be further expanded in a low pressure turbine.
In this respect, the gas turbine plant may include one (or more) sequential combustion gas turbine units, i.e., gas turbines having a compressor, a first combustion chamber, a high pressure turbine, a second combustion chamber, and a low pressure turbine.
Alternatively, the gas turbine plant may include two (or more) superimposed gas turbine units, such that the hot gases discharged from a first gas turbine unit are supplied to a second gas turbine unit.
Thus, in this alternative embodiment the gas turbine plant has a compressor, a first combustion chamber, and a high pressure turbine (defining a first gas turbine unit); the hot gases discharged from the first gas turbine unit are then supplied to a second gas turbine unit having a compressor, a combustion chamber, and a low pressure turbine.
2. Brief Description of the Related Art
As known in the art, a way to increase the power output of a gas turbine plant is to increase the mass flow rate circulating within the turbines.
Typically, the mass flow rate is increased by injecting water into the combustion chambers; usually, water is injected by high fogging injection or fogging and steam injection into the combustion chambers.
Water injection into the combustion chambers greatly influences the combustion conditions, because the flame temperature decreases, causing larger non-combusted product generation (such as CO or UHC) and lower NOx generation.
In addition, the flame stability is also influenced and, in particular, a reduced flame temperature causes a less stable flame.
Therefore, injecting an excessive amount of water into a combustion chamber, in addition to a too large non-combusted product generation (CO, UHC, which is a negative effect) and a low NOx generation (which is a positive effect), could cause the stability limit to be reached and also overcome, this leading to very unstable combustion with generation of pulsations that cause mechanical stress and reduced lifetime for the turbines and noisy operation.
Therefore the amount of water that is injected into the combustion chambers during operation must be controlled and must be limited, such that the non-combusted products (CO and UHC) generation is not excessive and pulsations are also limited.
Typically, the amount of water injected into each combustion chamber is regulated on the basis of the non-combusted product generation (such as CO or UHC) and NOx generation, because these limits are stricter than the flame stability limit.
Clearly this regulation, limiting the amount of water that can be injected into the combustion chambers, limits the power increase that can be achieved via water injection.